Single-Layer PtI2: A Multifunctional Material with Promising

Subscriber access provided by GUILFORD COLLEGE

Energy, Environmental, and Catalysis Applications

Single-Layer PtI2: A Multifunctional Material with Promising Photocatalysis toward Oxygen Evolution Reaction and Negative Poisson’s Ratio Shiying Shen, Yandong Ma, Hao Wang, Baibiao Huang, and Ying Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07300 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Single-Layer PtI2: A Multifunctional Material with Promising Photocatalysis toward Oxygen Evolution Reaction and Negative Poisson’s Ratio

Shiying Shen, Yandong Ma*, Hao Wang, Baibiao Huang, Ying Dai* School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Shandanan Street 27, Jinan 250100, People’s Republic of China *Corresponding Author: [email protected]; [email protected]

ABSTRACT: Exploring novel two-dimensional multifunctional materials with distinguished properties lies at the core of materials innovation. Here, we identify a hitherto neglected single-layer PtI2, which can readily be exfoliated from its bulk experimentally, with appealing multifunctional properties through first-principles calculations. Single-layer PtI2 is an indirect-gap semiconductor with a band gap of 2.56 eV, and its band gap is extremely robust against external strain. We find it is a promising two-dimensional photocatalyst, exhibiting outstanding catalytic performances toward oxygen evolution reactions, with the water oxidation reaction occurring spontaneously under light irradiation. Moreover, we unveil that single-layer PtI2 is a long-sought auxetic material with a negative Poisson’s ratio of -0.54, arising from its particular puckered hinge crystal. Our findings highlight single-layer PtI2 as a promising multifunctional material for nanoscale mechanical and photocatalytic applications.

KEYWORDS: single-layer, PtI2, multifunctional material, oxygen evolution reaction, negative Poisson’s ratio

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

1. Introduction Two-dimensional (2D) materials, allowing fundamentally new properties and physics, has stimulated intensive research interest.1-3 Except for various properties in the 2D world, of particular interest are photocatalysis and auxeticty. Compared to threedimensional photocatalytic materials, 2D materials exhibit large specific surface area, which can provide as many surface reactive sites as possible. Meanwhile, the ultrathin nature of 2D materials allows water splitting reactions to occur immediately, beneficial for decreasing the recombination rate of photo-generated carriers. To date, a large amount of 2D materials have been investigated due to their potential photocatalytic water-splitting applications.4-18 Unfortunately, many of them suffer from poor longterm durability and virtually inactive for overall water-splitting.19,

20

And another

challenge in this field is to reduce the substantial energy loss caused by the high overpotential in oxygen evolution reaction (OER). Therefore, it is of profound interest to explore new 2D photocatalysts, specially with low over-potential to realize water oxidation half reaction without using sacrificial reagents and co-catalysts. The Poisson’s ratio depicts the resultant strain in the longitudinal direction for one material under lateral stress. Most materials exhibit positive Poisson’s ratio (PPR), namely, one will observe a compression along longitudinal direction upon imposing a tensile strain in transverse direction and vice versa. Compared with 2D crystals with a PPR, those harboring the intriguing negative Poisson’s ratio (NPR), also termed as auxetic materials, generate more recent interest because of their unusual behaviors including shear resistance, toughness, vibration absorption and enhanced sound. The auxetic materials provide enormous potential in fields of biomedicine,21 fasteners,22 and protective equipments.23 Nevertheless, up to now, only a few studies on 2D auxetic


Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials have been reported in literature.24-31 And even for these few proposed 2D auxetic systems, the reported NPR values are rather small,24-26 which severely hampers their practical applications. Bulk PtI2 has been known since 1986. Thiele et al. demonstrated that bulk PtI2 features a layered structure with a monoklin geometry.32 In the present work, based on first-principles calculations, we identify that single-layer (SL) PtI2 is a compelling 2D multifunctional crystal with both photocatalysis and a NPR. The exfoliation energy for SL PtI2 is 0.47 Jm-2, comparable to those of graphite and MoS2,33 which suggests that SL PtI2 can be readily exfoliated from its bulk. SL PtI2 is an intrinsic semiconductor with an indirect band gap of 2.56 eV. Notably, it exhibits superior OER catalytic activity, making it promising for applications in photocatalytic water-splitting. And more interestingly, SL PtI2 possesses a unique NPR with a value as large as -0.54. Our results thus demonstrate that SL PtI2 holds simultaneously two functions, OER photocatalysis and NPR, which are highly desirable for multifunctional applications.

2. Computational Methods Our first-principles calculations are carried out using density-functional theory (DFT) as implemented in Vienna ab initio Simulation Package (VASP).34,

35

The electron

exchange-correlation functional is described by the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerh parametrization.36-38 The cutoff energy is set to 450 eV. Brillouin zone is sampled using 5×5×1 Monkhorst−Pack k-grids. A vacuum space of 20 Å is adopted to prevent interlayer interactions. The van der Waals interactions are described using DFT+D3.39 Atom positions and lattice parameters are fully optimized until the force on each atom is less than 0.01 eVÅ-1. Electronic structures are calculated employing Heyd-Scuseria-Ernzerhof hybrid (HSE06) functional.40 As shown in Figure S1a in Supporting Information (SI), HSE06 and HSE06+SOC give similar results, and therefore the spin-orbit coupling is not considered here. Phonon spectra are calculated based on the supercell approach using PHONOPY code.41 AIMD simulations are carried out at for 10 ps under 300 K, and time step is set to 3 fs. In aqueous solution, the OER process generally involves four-electron oxidation



Page 4 of 19

steps, which can be expressed as follows: ∗ + H2O→OH ∗ + H + +e

(1)

OH ∗ →O ∗ + H + +e

(2)

O ∗ + H2O→OOH ∗ + H + +e

(3)

OOH ∗ → ∗ + O2 + H + +e

(4)

Here, * denotes the catalyst, and OH*, O* and OOH* denote the adsorbed intermediates. Gibbs free-energy change ΔG is calculated using the method proposed by Nørskov et al.,42 which is defined as: ∆G = ∆E + ∆EZPE ―T∆S + ∆GPH +∆GU. Here, ∆E corresponds to DFT energy difference between the free standing and adsorbed states of reaction intermediates; ∆EZPE is the change in zero-point energy, where zeropoint energy is taken from the vibrational frequency calculations; ΔS is the change in entropy. And entropy can be taken from the NIST database. Room temperature (T = 298.15 K) is adopted here. The calculated EZPE and S are displayed in Table S1. ∆GPH = kBT × pH × ln10 represents the free energy correction of pH. ∆G𝑈 is calculated with ∆G𝑈 = ―eU. Here, U is the electrode potential with respect to the standard hydrogen electrode (SHE). Except the band structure calculations which are based on the HSE06 functional, all the calculations are performed using the PBE functional. More details for computational method are shown in SI.

3. Results and Discussion As shown in Figure S1b, bulk PtI2 crystallizes in a monoclinic structure with a space group P21/c (no. 14), and it adopts a layered structure with van der Waals layers stacked along the z direction. The lattice constants of bulk PtI2 are a = 8.48 Å, b = 6.84 Å, and c = 6.64 Å, which agree with previous experimental results.32 Each slab displays a wavy structure with a finite thickness of 6.27 Å. In each slab, the atoms are covalently bound. The oxidation states of Pt and I are +2 and -1, respectively, which facilitates the charge balance. The band structure of bulk PtI2 is shown in Figure S1c, from which we can


Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


see that bulk PtI2 is a semiconductor with an indirect band gap of 1.80 eV. The crystal structure of SL PtI2 is displayed in Figure 1. The unit cell of SL PtI2 consists of four Pt atoms and eight I atoms, forming a rectangular lattice. The lattice parameters are found to be a = 9.14 Å, b = 7.14 Å. Each Pt atom coordinates with four I atoms, forming a quadrilateral planar geometry (PtI4), and each I atom in the middle coordinates with two Pt atoms. Overall, the atoms are stacked in five layers, resulting in a buckling height of 5.86 Å, featuring a unique washboard-like morphology; see Figure 1a. To our knowledge, 2D sheet with such a remarkable wrinkle has yet to be reported in the previous literatures, which can result in fascinating behaviors, as we will show below.

Figure 1. (a) Crystal structures of SL PtI2 from different side views, with purple dashed line indicating the unit cell. Black (purple) balls represent Pt (I) atoms, respectively. (b) Phonon bands of SL PtI2. (c) The fluctuations of total energy (purple) and temperature T (green) during AIMD simulations at 300 K. Insets in (c) show the snapshots of SL PtI2 at 0 and 10 ps of MD simulation at 300 K. (d) Simulated STM image (+1.0 V) for SL PtI2. Insert in (d) shows the sliced plane (solid line).



To estimate the experimental feasibility for exfoliating SL PtI2, we examine its cleavage energy by peeling off the top layer from the five-layer slab, and the obtained cleavage energy is ~0.47 Jm-2. This is a relative small value for 2D crystals, which is comparable to those of graphite (0.37 Jm-2) and MoS2 (0.42 Jm-2). Therefore, the micromechanical cleavage43 or liquid exfoliation44 of SL PtI2 is accessible. To evaluate the stability of SL PtI2, we plot its phonon spectra in Figure 1b. The absence of imaginary frequency over the entire Brillouin zone is observed, indicating the dynamical stability of SL PtI2. Besides, the highest vibration frequency is 193 cm1,

which is much smaller than that of 2D MoS2 (473 cm-1),45 reflecting the mechanical

soft of the covalent Pt-I bonds. The elastic constants of SL PtI2 (C11 =2.12 N/m, C22 = 15.62 N/m, C12 = 1.28 N/m, and C66 = 0.42 N/m) obey the criteria for being stable 2D crystal (C11 > |C12|, C66 > 0),46 suggesting its mechanical stability. Furthermore, the thermal stability of SL PtI2 is examined via AIMD simulations in which the temperature of the system is controlled at 300 and 500 K. As shown in Figure 1c and Figure S2, its structure is not disrupted throughout the 10 ps dynamical simulation up to 500 K, indicating the thermal stability of SL PtI2. And to facilitate its observation in experiment, the STM image of SL PtI2 is simulated and plotted in Figure 1d. Based on the above results, we conclude that SL PtI2 is stable. Figure 2a plots the band structure of SL PtI2. Clearly, it is semiconducting with an indirect gap of 2.56 eV., with its valence band maximum (VBM) locating at the M point and conduction band minimum (CBM) lying between the Γ and Y points. Notably, the direct band gap at the M point (2.65 eV) is quite close to the indirect band gap, with a slight difference of 0.09 eV. As revealed by the projected density of states (PDOS) shown in Figure 2a, the major contribution of the states near the Fermi level derives from Pt-d and I-p. This is consistent with the partial charge density distributions (Figure S3). Upon increasing single-layer to double-layer, the CBM moves to the A point and the VBM shifts to the Y point, forming an indirect band gap of 2.20 eV. However, the direct band gap at the Y point for the double-layer PtI2 is 2.22 eV and slightly larger than the direct band gap. Therefore, double-layer PtI2 exhibits a quasidirect band gap; see Figure 2b.


Page 6 of 19

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Band structure and PDOS of single-layer PtI2. (b) Band structure and PDOS of double-layer PtI2. The energy level positions are aligned relative to vacuum level. (c) Evaluations of band edge positions of SL PtI2 with the strain. (d) Optical absorbances of SL PtI2.

It is known that 2D semiconductors harboring a band gap ranging from 1.5 eV to 3.0 eV show high possibility to be photocatalysts for water splitting.47,

48

Being a

photocatalyst for water-splitting, the band edges should straddle the water redox potentials. In detail, the CBM should lie above the reduction potential of H+/H2 (-4.44 eV), while the VBM should locate below the oxidation potential of O2/H2O (-5.67 eV). It should be noted SL PtI2 exhibits an indirect band gap, which may be not ideal for light adsorption. In fact, the typical photocatalyst TiO2 is also an indirect-gap semiconductor. So it is expected that such indirect band gap feature will not degrade the photocatalytic performance of SL PtI2 very much. As depicted in Figure 2a, strikingly, the CBM and VBM of SL PtI2 locate at the positions of -3.93 and -6.49 eV, respectively, at pH = 0. While for double-layer PtI2 (see Figure 2b), the CBM and



VBM locate at the positions of -4.08 and -6.28 eV, respectively. Both systems thus could well satisfy the thermodynamic requirements for overall water splitting, suggesting that they are potential 2D photocatalysts. For realistic applications, external or internal strain is unavoidable, which may deform their photocatalytic nature. Accordingly, we study the band structures of SL PtI2 under various strain in detail (Figure S4), which manifests that the strain would not cause an indirect-to-direct bandgap transition. As shown in Figure 2c, the band gap of SL PtI2 shows a monotonic decrease tendency with increasing strain: the band gap is reduced to 2.45 eV under tensile strain 3%, while it reaches 2.66 eV under compressive strain -3%. In other words, the band gap is only slightly affected by the strain due to the structural backbone with a large wrinkle. Also, the strain effect on the band edge positions are investigated. From Figure 2c, we can see that both the VBM and CBM of SL PtI2 shift down with increasing (decreasing) the tensile (compressive) strain, but always straddling the water redox potentials. Therefore, the photocatalysic water splitting in SL PtI2 is robust against strain, which is beneficial for its practical photocatalytic applications. To be a promising photocatalyst, the optical absorption is vital. To this end, we characterize their optical performances using the GW approximation in conjunction with Bethe-Salpeter equation. As shown in Figure 2d, both SL and double-layer PtI2 exhibit a remarkably high light harvesting capability in both visible and ultraviolet spectrum ranges. Particularly, the absorption coefficients for SL PtI2 can research up to about 106 cm-1, which is even comparable to that of organic perovskite solar cells.49, 50 Such remarkable absorbance coefficient and the broad absorbance range make us more confidential that SL and double-layer PtI2 will be attractive candidates for photocatalytic water splitting. In addition, both systems exhibit a unique anisotropic optical absorption behavior between the x and y directions, which is attributed to their unique structures.


Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) Atomic structures of intermediates involved in the water oxidation reaction on SL PtI2, with red (green) balls indicating O (H) atoms. (b) ΔG for the four elementary steps of water oxidation reaction on the surface of SL PtI2. U= 2.46 V corresponds to the potentials provided by light irradiation at pH = 7. As over-potential of OER is critical for the light-driven conversion of water to O2, we then explore the OER process on SL PtI2 according to the four electron steps [see eq. (1) - (4)], in which various reaction intermediates are involved. And each electron step is accompanied by releasing an H+ cation and an electron. The potential of photogenerated holes for water oxidation (Uox) is taken as the energy difference between the hydrogen reduction potential and the VBM. The higher Uox evokes the better photoinduced corrosion resistance. Uox for SL PtI2 is found to be Uox = 2.05 V at pH=0. Note that Uox is the electrode potential relative to the standard hydrogen electrode, it can be flexibly modulated by pH (see SI). In what follows, we mainly concentrate on the neutral environment (pH = 7) with Uox = 2.46 V, which manifests a better photocatalytic activity. The crystal structures of intermediates involved in the water oxidation reaction on the surface of SL PtI2 are illustrated in Figure 3a, and the corresponding free-energy diagrams are summarized in Figure 3b. The whole OER contains the following elementary steps: (i) water molecule experiences deprotonation and dissociates to OH*; (ii) OH* further releases an H+ and an electron and dissociates into O*; (iii) O* reacts with another water molecule and forms OOH*; (iv) OOH* deprotonates to form a O2 molecule, which is then released. As shown in Figure S5, there are three different adsorption sites for OH adsorption (namely, sites Pt, I and V). The corresponding ΔGOH* is listed in Table S2, from which it can be seen that OH* is prone to be anchored on the Pt site. And from Figure 3b we can see that without considering any light irradiation (Uox = 0 V), the reaction steps for the formations of OH* and OOH* are



endothermic with a ΔG of 1.93 and 2.01 eV, respectively, while the other two elementary steps are exothermic. Therefore, the OER is hard to proceed spontaneously on the surface of SL PtI2 without light irradiation (Uox = 0 V). An external potential is required to drive the whole reaction on SL PtI2. The overpotential η of OER is estimated by:

𝜂=

𝑚𝑎𝑥 [𝛥𝐺1,𝛥𝐺2,𝛥𝐺3,𝛥𝐺4,] 𝑒

―1.23 + 0.059 × 𝑝𝐻.

For SL PtI2, the conversion of O* to OOH* is the rate-limiting step, with a limiting potential of 2.01 eV. Based on the above relation, the water oxidation overpotential for SL PtI2 is calculated to be 1.19 eV. Upon including light irradiation (Uox = 2.46 V), the water oxidation overpotential of SL PtI2 can be easily overcome. As shown in Figure 3b, all the elementary steps are downhill in energy with considering light irradiation, which suggests that O2 can be voluntarily generated from SL PtI2 in the neutral environment. Furthermore, the ΔGOH is found to be 1.93 eV, which strongly implies that this catalyst is hard to get poisoned.51 These encouraging results firmly illustrate that SL PtI2 is an compelling OER catalyst, exhibiting an outstanding catalytic activity even without using any sacrificial reagents or co-catalysts.

Figure 4. Polar diagrams of (a) Poisson’s ratio of SL PtI2. Mechanical response induced by applied strain in (b) x- and (c) y-directions.

Aside from the photocatalytic performances, we also inspected the mechanical behaviors of SL PtI2 in view of its unique washboard-like morphology. The orientationdependent Poisson’s ratio υ(θ) and Young’s modulus Y(θ) of SL PtI2 are shown in Figure 4a and S6. Clearly, we can observe that SL PtI2 exhibits an anisotropic mechanical behavior. The maximal Young’s modulus of 14.85 N/m is obtained along the y axis, while the minimum one of 2.02 N/m occurs along the x axis. These values


Page 10 of 19

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are smaller than those of MoS2 (129 N/m),52 graphene (340 N/m),53 and h-BN (318 N/m).54 Such small values of Young’s modulus suggest the mechanical flexible of SL PtI2. More importantly, the small Young’s modulus in SL PtI2 occurs along both x and y direction, rendering it the first material with bi-directional flexibility.

Table 1. The reported negative Poisson’s ratios (ν) in 2D auxetic materials System

δ-2D silica

δ-phosphorene

Be5C2

AsN

SnSe

Ag2S

PtI2

ν

-0.11

-0.267

-0.16

-0.176

-0.17

-0.12

-0.54

Poisson’s ratio can be expressed as follows: υ = ― 𝜀𝑡𝑟𝑎𝑛𝑠𝑣𝑒𝑟𝑠𝑒 𝜀𝑎𝑥𝑖𝑎𝑙, which characterizes the material’s resultant strain in the longitudinal direction for a material under lateral stress. Materials usually possess a positive Poisson ratio, and the materials with a NPR are extremely scarce.55 Remarkably, as shown in Figure 4a, SL PtI2 indeed expands along special directions when imposing an external tensile stress in the vertical diagonal direction, suggesting the promising auxetic effect. More importantly, the NPR of SL PtI2 reaches the highest value of -0.54 at 55°. For comparison, the typical negative Poisson’s ratio values of 2D auxetic materials reported in previous works are summarized in Table 1.24-29 Encouragingly, the negative Poisson’s ratio value of SL PtI2 is larger than those of previously known 2D auxetic systems. To further investigate the mechanical properties of SL PtI2, the uniaxial strains along x and y directions are applied respectively. The Poisson’s ratio can be obtained by fitting ε𝑟𝑠 = ― υ1ε𝑠 + υ2 ε2𝑠 + υ3ε3𝑠 , where ε𝑟𝑠 and ε𝑠 indicate resultant strain and strain, and υ1 could be regarded as the Poisson’s ratio. As shown in Figure 4(b, c), when stretched (compressed) along the x direction, SL PtI2 contracts (expands) along the y direction, and vice versa. This clearly suggests SL PtI2 exhibits an in-plane PPR along x and y directions, agreeing well with the above analysis. While for NPR along the diagonal directions, we do not consider it here as it is hard to construct the corresponding structure model. Moreover, the out-of-plane NPR (-0.24) is also observed in SL PtI2 when applying strain along the y direction. Such NPR phenomenon in SL PtI2 is attributed to the unique buckling hinge-like structure, which is similar to the cases of phosphorene and Pd2Se3.56-58 The presence of NPR definitely would entail SL PtI2 being



highly useful for mechanical applications.

4. Conclusion In summary, we identify that SL PtI2 is a promising multifunctional material with both photocatalysis and a NPR. We find that SL PtI2 is an indirect gap semiconductor, exhibiting a superior OER catalytic activity, making it promising for applications in photocatalytic water-splitting. Meanwhile, SL PtI2 features a unique NPR with a value as large as -0.54 because of the hinged geometric structure. Moreover, the small exfoliation energy attests to its experimental viability. All of these appealing properties expect that SL PtI2 could be realized in the laboratory and be utilized as a promising multifunctional material in the near future.

Associated Content Supporting Information: Relevant Calculation details of the OER process; optimized structures and total energy difference for other phases of SL PtI2; crystal structure and band structure of bulk PtI2; AIMD simulations at 500 K, partial charge density, different adsorption sites, Young’s modulus of SL PtI2; band structures of SL PtI2 under different strains; data for free energy diagram, Gibbs free energy in different surface sites.

Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 11804190), Shandong Provincial Natural Science Foundation of China (Nos. ZR2019QA011 and ZR2019MEM013), Qilu Young Scholar Program of Shandong University, and Taishan Scholar Program of Shandong Province, and Youth Science and Technology Talents Enrollment Project of Shandong Province. References 1.

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S.

V.; Grigorieva, I. V. ; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon


Page 12 of 19

Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Films. Science 2004, 306, 666-669. 2.

Matte, H. S.; Gomathi, A.; Manna, A. K.; Late, D. J.; Datta, R.; Pati, S. K.; Rao,

C. N. MoS2 and WS2 Analogues of Graphene. Angew. Chem., Int. Ed. 2010, 49, 40594062. 3.

Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tomanek, D.; Ye, P. D. Phosphorene:

An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033-4041. 4.

She, X.; Wu, J.; Zhong, J.; Xu, H.; Yang, Y.; Vajtai, R.; Lou, J.; Liu, Y.; Du, D.;

Li, H.; Ajayan, P. M. Oxygenated Monolayer Carbon Nitride for Excellent Photocatalytic Hydrogen Evolution and External Quantum Efficiency. Nano Energy 2016, 27, 138-146. 5.

Su, T.; Shao, Q.; Qin, Z.; Guo, Z.; Wu, Z. Role of Interfaces in Two-Dimensional

Photocatalyst for Water Splitting. ACS Catal. 2018, 8, 2253-2276. 6.

Krishnamoorthy, K.; Mohan, R.; Kim, S. J. Graphene Oxide as a Photocatalytic

Material. Appl. Phys. Lett. 2011, 98, 244101. 7.

Liu, C.; Wang, L.; Tang, Y.; Luo, S.; Liu, Y.; Zhang, S.; Zeng, Y.; Xu, Y. Vertical

Single or Few-Layer MoS2 Nanosheets Rooting into TiO2 Nanofibers for Highly Efficient Photocatalytic Hydrogen Evolution. Appl. Catal., B 2015, 164, 1-9. 8.

Dong, X.; Cheng, F. Recent Development in Exfoliated Two-Dimensional g-C3N4

Nanosheets for Photocatalytic Applications. J. Mater. Chem. A 2015, 3, 23642-23652. 9.

Zhao, P.; Ma, Y.; Lv, X.; Li, M.; Huang, B.; Dai, Y. Two-Dimensional III2-VI3

Materials: Promising Photocatalysts for Overall Water Splitting under Infrared Light Spectrum. Nano Energy 2018, 51, 533-538. 10. Liang, Y.; Long, C.; Li, J.; Jin, H.; Huang, B.; Dai, Y. InSe Monolayer: Promising Cocatalyst of g‑C3N4 for Water Splitting under Visible Light. ACS Appl. Energy Mater. 2018, 1, 5394–5401.



11. Han, Q.; Wang, B.; Gao, J.; Cheng, Z.; Zhao, Y.; Zhang, Z.; Qu, L. Atomically Thin Mesoporous Nanomesh of Graphitic C3N4 for High-Efficiency Photocatalytic Hydrogen Evolution. ACS Nano 2016, 10, 2745-2751. 12. Jing, Y.; Heine, T. Two-Dimensional Pd3P2S8 Semiconductors as Photocatalysts for the Solar-driven Oxygen Evolution Reaction: A Theoretical Investigation. J. Mater. Chem. A 2018, 46, 23495–23501. 13. Qiao, M.; Liu, J.; Wang, Y.; Li, Y.; Chen, Z. PdSeO3 Monolayer: Promising Inorganic 2D Photocatalyst for Direct Overall Water Splitting Without Using Sacrificial Reagents and Cocatalysts. J. Am. Chem. Soc. 2018, 140, 12256-12262. 14. Liang, Y.; Dai, Y.; Ma, Y.; Ju, L.; Wei, W.; Huang, B. Novel Titanium Nitride Halide TiNX (X = F, Cl, Br) Monolayers: Potential Materials for Highly Efficient Excitonic Solar Cells. J. Mater. Chem. A 2018, 6, 2073-2080. 15. Saraf, D.; Chakraborty, S.; Kshirsagar, A.; Ahuja, R. In Pursuit of Bifunctional Catalytic Activity in PdS2 Pseudo-Monolayer through Reaction Coordinate Mapping. Nano Energy 2018, 49, 283-289. 16. Zhang, X.; Chen, A.; Zhang, Z.; Jiao, M.; Zhou, Z. Rational Design of C2N-Based Type-II Heterojunctions for Overall Photocatalytic Water Splitting. Nanoscale Adv. 2019, 1, 154-161. 17. Zhou, S.; Liu, N.; Wang, Z.; Zhao, J. Nitrogen-Doped Graphene on Transition Metal Substrates as Efficient Bifunctional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions. ACS Appl. Mater. Interfaces 2017, 9, 22578-22587. 18. Zhou, S.; Yang, X.; Pei, W.; Liu, N.; Zhao, J. Heterostructures of MXenes and NDoped Graphene as Highly Active Bifunctional Electrocatalysts. Nanoscale 2018, 10, 10876-10883. 19. Gao, G.; Jiao, Y.; Ma, F.; Jiao, Y.; Waclawik, E.; Du, A. Charge Mediated Semiconducting-to-Metallic Phase Transition in Molybdenum Disulfide Monolayer


Page 14 of 19

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Hydrogen Evolution Reaction in New 1T′ Phase. J. Phys. Chem. C 2015, 119, 13124-13128. 20. Zhang, G.; Lan, Z. A.; Lin, L.; Lin, S.; Wang, X. Overall Water Splitting by Pt/gC3N4 Photocatalysts without Using Sacrificial Agents. Chem. Sci. 2016, 7, 3062-3066. 21. Scarpa, F. Auxetic Materials for Bioprostheses. IEEE Signal Process. Mag. 2008, 25, 128-126. 22. Choi, J.; Lakes, R. Design of a Fastener Based on Negative Poisson's Ratio Foam. Cell. Polym. 1991, 10, 205-212. 23. Sanami, M.; Ravirala, N.; Alderson, K.; Alderson, A. Auxetic Materials for Sports Applications. Proc. Eng. 2014, 72, 453-458. 24. Gao, Z.; Dong, X.; Li, N.; Ren, J. Novel Two-Dimensional Silicon Dioxide with in-Plane Negative Poisson’s Ratio. Nano Lett. 2017, 17, 772-777. 25. Wang, H.; Li, X.; Li, P.; Yang, J. δ-Phosphorene: A Two Dimensional Material with a Highly Negative Poisson’s Ratio. Nanoscale 2017, 9, 850-855. 26. Wang, Y.; Li, F.; Li, Y.; Chen, Z. Semi-Metallic Be5C2 Monolayer Global Minimum with Quasi-Planar Pentacoordinate Carbons and Negative Poisson’s Ratio. Nat. Commun. 2016, 7, 11488. 27. Xiao, W. Z.; Xiao, G.; Rong, Q. Y.; Wang, L. L. Theoretical Discovery of Novel Two-Dimensional VA-N Binary Compounds with Auxiticity. Phys. Chem. Chem. Phys. 2018, 20, 22027-22037. 28. Zhang, L. C.; Qin, G.; Fang, W. Z.; Cui, H. J.; Zheng, Q. R.; Yan, Q. B.; Su, G. Tinselenidene: A Two-Dimensional Auxetic Material with Ultralow Lattice Thermal Conductivity and Ultrahigh Hole Mobility. Sci. Rep. 2016, 6, 19830. 29. Peng, R.; Ma, Y.; He, Z.; Huang, B.; Kou, L.; Dai, Y. Single-Layer Ag2S: A TwoDimensional Bidirectional Auxetic Semiconductor. Nano Lett. 2019, 19, 1227-1233.



30. Meng, L.; Zhang, Y.; Zhou, M.; Zhang, J.; Zhou, X.; Ni, S.; Wu, W. Unique Zigzag-Shaped Buckling Zn2C Monolayer with Strain-Tunable Band Gap and Negative Poisson Ratio. Inorg. Chem. 2018, 57, 1958-1963. 31. Meng, L. B.; Ni, S.; Zhang, Y. J.; Li, B.; Zhou, X. W.; Wu, W. D. TwoDimensional Zigzag-Shaped Cd2C Monolayer with a Desirable Bandgap and High Carrier Mobility. J. Mater. Chem. C, 2018, 6, 9175-9180. 32. Thiele, G.; Weigl. W.; Wochner, H. Die Platiniodide PtI2, und Pt3I8. 1986, 539, 141-153. 33. Bjorkman, T.; Gulans, A.; Krasheninnikov, A. V.; Nieminen, R. M. van der Waals Bonding in Layered Compounds from Advanced Density-Functional First-Principles Calculations. Phys. Rev. Lett. 2012, 108, 235502. 34. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 35. Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244-13249. 36. Kresse, G. F.; Furthmuller, J. Efficient Iterative Schemes for Ab initio TotalEnergy Calculations using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 1116911186. 37. Blöchl, P. E. Projector Augmented-Wave Method Phys. Rev. B 1994, 50, 1795317979.38. 38. Perdew, J. P.; Zunger, A. Self-Interaction Correction to Density-Functional Approximations for Many-Electron Systems. Phys. Rev. B 1981, 23, 5048-5079. 39. Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456-1465. 40. Jochen Heyd, G. E. S., and Matthias Ernzerhof. Hybrid Functionals Based on a


Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207-8215. 41. Togo, A.; Tanaka, I. First Principles Phonon Calculations in Materials Science. Scri. Mater. 2015, 108, 1-5. 42. Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892. 43. Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Singleand Few-Layer MoS2. ACS Nano 2010, 4, 2695-2700. 44. Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H. Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. TwoDimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571. 45. Molina-Sánchez, A.; Wirtz, L. Phonons in Single-layer and Few-Layer MoS2 and WS2. Phys. Rev. B 2011, 84, 155413. 46. Wu, Z.; Zhao, E.; Xiang, H.; Hao, X.; Liu, X.; Meng, J. Crystal Structures and Elastic Properties of Superhard IrN2 and IrN3 from First Principles. Phys. Rev. B 2007, 76, 054115. 47. Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-502. 48. Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. A Review and Recent Developments in Photocatalytic Water-Splitting Using TiO2 for Hydrogen Production. Renew. Sustain. Energy Rev. 2007, 11, 401-425.



49. Shirayama, M.; Kadowaki, H.; Miyadera, T.; Sugita, T.; Tamakoshi, M.; Kato, M.; Fujiseki, T.; Murata, D.; Hara, S.; Murakami, T. N.; Fujimoto, S.; Chikamatsu, M.; Fujiwara, H. Optical Transitions in Hybrid Perovskite Solar Cells: Ellipsometry, Density Functional Theory, and Quantum Efficiency Analyses for CH3NH3PbI3. Phys. Rev. Appl. 2016, 5, 014012. 50. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Erformance Inorganic-Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. 51. Ling, C.; Shi, L.; Ouyang, Y.; Zeng, X. C.; Wang, J. Nanosheet Supported SingleMetal Atom Bifunctional Catalyst for Overall Water Splitting. Nano Lett. 2017, 17, 5133-5139. 52. Cooper, R. C.; Lee, C.; Marianetti, C. A.; Wei, X.; Hone, J.; Kysar, J. W. Nonlinear Elastic Behavior of Two-Dimensional Molybdenum Disulfide. Phys. Rev. B 2013, 87, 035423. 53. Andrew, R. C.; Mapasha, R. E.; Ukpong, A. M.; Chetty, N. Mechanical Properties of Graphene and Boronitrene. Phys. Rev. B 2012, 85, 125428. 54. Michel, K. H.; Verberck, B. Theory of Elastic and Piezoelectric Effects in TwoDimensional Hexagonal Boron Nitride. Phys. Rev. B 2009, 80, 224301. 55. Evans, K. E.; Alderson, A. Auxetic Materials: Functional Materials and Structures from Lateral Thinking. Adv. Mater. 2000, 12, 617-628. 56. Jiang, J. W.; Park, H. S. Negative Poisson’s Ratio in Single-Layer Black Phosphorus. Nat. Commun. 2014, 5, 4727. 57. Lv, P.; Tang, G.; Yang, C.; Deng, J.; Liu, Y.; Wang, X.; Wang, X.; Hong, J. HalfMetallicity in Two-Dimensional Co2Se3 Monolayer with Superior Mechanical Flexibility. 2D Mater. 2018, 5, 045026. 58. Peng, R.; Ma, Y.; Wu, Q.; Huang, B.; Dai, Y. Two-dimensional materials with


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intrinsic auxeticity: progress and perspectives. Nanoscale 2019, 11, 11413-11428.


Single-Layer PtI2: A Multifunctional Material with Promising

Recommend Documents